Blender video in which a scene is first untextured and then shows the textures as the video goes on
A simple version of this effect could be created using a combination of render layers and keyed compositing effects:



Step 1

Under the Render Layers panel, enable AO and Environment passes.



Step 2

In the Compositer, create this node setup and keyframe the Mix node property highlighted below to go from 1 to 0 over the course of your animation, or over whatever period you want your materials to appear.



For very complex scenes you may need to play around with rendering more layers and mixing in different nodes, but this method should get you most of the way there for a basic fade in effect.